If you're 13, I wouldn't go anywhere above 15... Reason being is that you are young and going through changes... The older people get, the more mature they are (in a perfect world). BUT, if an older guy is interested in you and vice versa, if he's more than a few years older than you, he's probably trying to take advantage of you because you're young and impressionable. You might look up to older guys and see them as responsible and mature, manly etc... But they see you as "she's willing to do what I say" and "I can make her do anything I want because she looks up to me"... Please be careful. When you enter your late teens into early 20's, the age difference doesn't matter as much, but always within reason. How do I know this? I'm an older guy and have had many younger girls crush on me, and I could have taken advantage of the situation and "fooled around" but instead, I gave them lectures on why they shouldn't feel the way they do because they WILL get hurt... I'm not that type of guy, but I'm speaking from my knowledge, some of my older friends have done this and it's very wrong. It will VERY LIKELY NOT LAST, more than very likely, I'd give it a 85% failure. They will tell you what you want to hear, do not believe them.
19 is low end. it is legal, but also usually moved out of the house and not a kid. 34 is high end. There are women older I find attractive, but, in complete, brutal, hold-nothing-back honesty? Young women are more attractive, and take longer to age. I'm sure at 40 I'd have no problems considering a 40 year old. At 32, I'd rather see if there is a 27 year old whose attention I can get.
Youngest... I don't know. Since I'm about to turn eighteen might as well keep it close for convenience, so 17, my age. Oldest... I honestly don't know. I always recognized the fact that anyone above a certain age would need me to get a little older, but that didn't stop me from falling for a 21 year old. Which is pitiful I know.